The invention relates to a desiccant for absorbing a moisture present in a hermetically sealed container, in particular, a desiccant suited for being filled in the hermetically sealed container, and an the desiccant-employing organic electroluminescence element.
In recent years, organic EL (electroluminescence) element, organic light emitting system such as organic EL display and organic EL illumination, and organic electronic device such as organic semiconductor and organic solar battery have been widely studied and investigated, and is expected to be applied to a wide variety of basic elements and other use.
The organic EL element comprises an organic EL layer of a thin film containing a luminescent organic compound sandwiched between a pair of electrodes, a cathode and an anode. The organic EL element is a spontaneous light-emitting element in which a hole and an electron are injected into the thin film and recombined to generate an exiton and which utilizes emission of light (fluorescence/phosphorescence) when the exiton is inactivated.
The most significant problem of the above-described organic EL element is to improve its durability and, above all, generation of non-light emitting portion called “dark spot” and the prevention of its growth. When the diameter of the dark spot grows in tens of micrometers (μm), the non-light emitting portion can be confirmed by visual observation. A principal cause of the dark spot generation is the influence of water and oxygen, and particularly water, which has been known to influence the element most seriously even in a trace amount.
It is, therefore, necessary to prevent water from penetrating into the organic EL element, which is presently represented by a hollow sealing structure as disclosed in JP 2002-33187 A. In accordance with the hollow sealing structure, the penetration of moisture or oxygen into the organic EL element can be controlled or prevented by sealing the organic EL element under dry inert gas atmosphere.
With reference to FIG. 10, the hollow sealing structure of the organic EL element 10 includes an organic EL portion 12, which includes a translucent insulating element substrate 11, an anode disposed on the substrate, and an organic layer disposed on the anode, and a cathode disposed on the organic layer. A drying film 15 is placed as a drying means on a sealing substrate 13 in a container, which is hermetically sealed by the element substrate 11, the sealing substrate 13 and a sealing agent 14.
However, in the above hollow sealing structure, the drying means such as inorganic drying sheet should be disposed inside the container so as to chemically or physically absorb moisture therein. In this case, the space for disposing the drying means should be provided in the container, thereby increasing the cost for manufacture. In addition, it is difficult to render the thickness of the drying film as the drying means uniform. As a result, in a case of top-emission type organic EL element in which light generated in the organic EL portion is guided via the translucent sealing substrate toward outside, there occurs a problem of distortion of display.
Moreover, nothing is in contact with the upper surface of the cathode, and heat can only be emitted via radiation and convection across a panel. For the reason as set forth above, heat cannot be sufficiently radiated away from, for example, organic EL for illumination. Furthermore, in the organic EL element having a size above a certain level, flexibility is generated by pressing its center portion. A sealing substrate is brought in contact with the organic EL portion, thereby being likely to break the organic EL portion.
In view of the above, there has been suggested a solid sealing structure in which the organic layer is embedded and sealed in resin, thereby slowing the speed of penetrating moisture and oxygen. This structure is advantageous in heat release properties and panel strength. See JP H08-236271 A.
Referring to FIG. 11, an organic EL element 20 having the solid sealing structure includes an organic EL portion 22, which has a translucent insulating element substrate 21, an anode disposed on the element substrate 21, an organic layer disposed on the anode, and a cathode disposed on the organic layer. Subsequently, a protective layer 23 is disposed on the organic EL portion 22. Subsequently, a sealing layer 24 is formed, and a container is hermetically sealed by a sealing substrate 25, which is configured to protect the organic layer from ambient air, and a sealing agent 26. The protective layer 23 is configured to mitigate stress on the cathode during the formation of the sealing layer 24, as well as, to suppress the reaction of chemical components used in the sealing layer 24 with the organic layer, thereby preventing possible damage on the organic layer.